Metallurgy



3,268,328 METALLURGY Maurice L. Torti, In, Boston, Mass, assignor to National Research Corporation, Cambridge, Mass, a corporation of Massachusetts No Drawing. Filed Nov. 3, 1964, Ser. No. 408,684 9 Claims. (Cl. 75-174) This application is in part a continuation of copending application Serial No. 244,955, filed December 17, 1962, now abandoned.

This invention relates to tantalum metal and alloys. More particularly the present invention relates to tantalum metal and alloys containing small but effective amounts of yttrium and the rare earths.

It is known that wrought tantalum metal and alloys which possess such qualities as higher recrystallization temperatures, resistance to grain coarsening at temperatures equal to or above the recrystallization temperature, uniformity of grain size and ductility would provide enhanced utility of tantalum metal and alloys in many technical and industrial applications. It is desirable to provide wrought metallic elements which have ductility in addition to strength at elevated temperatures. In electrical applications, where small diameter filament and electrode lead wires are used, grain growth and resultant embrittlement of the wires when fabricated at temperatures in excess of the recrystallization temperature present serious problems. Grain growth in fine wire filaments and electrode leads results in large grains which may extend completely across the diameter of the wire. These large grains have a tendency to slip at the grain boundaries thus reducing or destroying the utility of the filament or electrode lead. Additionally coarse grain structures have a tendency to become embrittled and crack when contaminated.

A combination of higher recrystallization temperature, resistance to grain growth or coarsening and uniformity of grain size in addition to ductility in wrought tantalum metal and alloys has proved diflicult to achieve and-considerable time and effort have been devoted in attempting to achieve such qualities for these metals. It is therefore apparent that ability to provide tantalum metal and alloys with these qualities would materially enhance the development of many types of technical, industrial and commercial equipment.

Accordingly, it is an object of the present invention to provide ductile wrought tantalum metal and alloys having improved recrystallization temperatures and resistance to grain growth or coarsening at elevated temperatures.

It is another object of the present invention to provide wrought tantalum metal and alloys having resistance to grain growth or coarsening at temperatures above the recrystallization temperature.

A further object of the present invention is to provide wrought tantalum metal and alloys which are useful in the production of metallic elements requiring a fine grain structure which is resistant to grain coarsening at elevated temperatures.

A still further object of the present invention is to provide wrought tantalum metal and alloys having greatly increased strength at elevated temperatures.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the products possessing the features, properties, and the relation of components and the process-involving the several steps and the relation and order of one or more of such steps with respect to each of the others which are exemplified in the following detailed disclosure and the scope of the application of which will be indicated in the claims.

In accordance with the present invention the foregoing objects are achieved by including in the tantalum metal and alloys a small but effective amount of rare earth (including yttrium and lanthanum) metals and oxides. In this manner, improved recrystallization temperatures, uniform small grain size, and resistance to grain growth are achieved for wrought tantalum metal and alloys.

The advantages of the present invention will be first discussed in connection with a preferred form thereof wherein about .001% to 1% yttrium is added to the tantalum metal or tantalum-base alloys.

The alloys of the present invention can be prepared in accordance with conventional procedures through recourse to known melting, casting, and metallurgical techniques. For example, measured amounts of the individual metal constituents can be melted together, solidified and remelted until homogeneity is obtained In the powder metallurgical route, measured amounts of the individual metal powder constituents are uniformly mixed and then sintered to provide a uniform mass. Numerous. types of conventional melting equipment may be usd to prepare the alloys of the present invention. The melting operation can, for example, be carried out in vacuum arc melting or electron beam furnaces. In vacuum arc melting furnaces, consumable, nonconsumable, or a combination of nonconsumable and consumable electrodes can be used. Where a consumable electrode is used, the preferred yttrium metal can be added to the electrode to be melted or the yttrium in particulate form such as chips or powder can be mixed with the melt stock from which the electrode is made. Also the yttrium can be separately added to the melt. Regardless of the type of furnacing means employed, care should be exercised to protect the molten metals from normal atmospheric contamination through contact with oxygen, nitrogen and the like. Thus, the melting operation should be carried out under inert conditions such as vacuum or an inert atmosphere.

Since yttrium has a relatively high vapor pressure, the amount of yttrium to be added initially in order to obtain the desired range of yttrium retained in the alloy ingot Will depend upon a number of conditions, such as for example the constituents to be melted, the melt rate, and the temperature of the molten metal. The initial yttrium addition will therefore be such as to obtain in the cast metal retained yttrium of about .001% to 1% as previously indicated. Preferably the yttrium content is between about .00l% and .l%.

The invention will now be described by way of the following non-limiting examples:

Example 1 In this example small pieces of tantalum metal in the form of machine turnings were mixed with sufficient yttrium metal chips to provide a total concentration of .O5% yttrium. The mixture of tantalum and yttrium was then hydrostatically pressed to form a consumable arc melting electrode. The electrode was then are melted in a conventional cold mold arc furnace. Power was supst 23, E966 3 plied by a bank of welding generators capable of supplying 5600 amperes. A mechanical booster high vacuum pump backed by a mechanical pump comprised the pumping system. A cold mold having an inner diameter of 4 inches was used. The tantalum-yttrium electrode was melted at 30 volts, 5000 amperes and at a pressure of .1 to 1 micron Hg abs. as measured in the furnace body. The alloy ingot produced contained .005 by weight yttrium metal, the balance being tantalum. The ingot was then cold rolled to greater than 90% reduction to provide a sheet of the alloy having a thickness of .020 inch. Sections of the rolled sheet were then vacuum annealed for 1 hour at temperatures ranging from between 1900 F. to 4000 F. The alloy sections were examined and the recrystallization temperature and grain size determined. By the term recrystallization temperature as used in the specification and claims, it is meant the approximate minimum temperature at which complete recrystallization of the cold worked metal to new equiaxed grains occurs within a time of one hour. By the term grain size as used in the specification and. claims, it is meant the number of grains as observed by comparison with a standard ASTM grain size chart at 100x magnification. The recrystallization temperature was 3700" F. and the grain size was ASTM 4.

Example 2 In this example sufiicient yttrium metal in the form of chips was mixed with tantalum powder to provide a total concentration of 0.2% yttrium. A consumable arc melting electrode was prepared and melted as in Example 1. The ingot produced was then fabricated into an electrode and remelted. The double melted alloy ingot formed contained .005 yttrium, the balance being tantalum. The recrystallization temperature and grain size was determined as in Example 1. The recrystallization temperature was 3990 F. with an ASTM grain size of 5.

Example 3 In this example a melt containing 02% yttrium, the balance being tantalum, was prepared by melting a mixture of yttrium metal and tantalum metal and then alternately solidifying and melting the alloy 3 to times to obtain a uniform ingot. The amount of yttrium initially added to the mixture was .l% by weight. The melting apparatus was a button-melting furnace made from a welding dry box having a permanent tungsten electrode and a water-cooled copper mold. The melting was carried out under an argon atmosphere. The alloy was then cold worked and tested for recrystallization temperature and grain size as in Example 1. At a temperature of 3700 F. this alloy was 75% recrystallized.

Example 4 Using the procedure of Example 3 an alloy of the following composition was prepared: 003% yttrium, the balance being tantalum. The amount of yttrium initially added to the melt was .2% by weight This alloy had a recrystallization temperature of 3700 F. and an ASTM grain size of 6.

Example 5 Using the procedure of Example 3 an alloy of the following composition was prepared: 005% yttrium, the balance being tantalum. The amount of yttrium initially added to the melt was .05 by weight. This alloy did not exhibit any recrystallization at the test temperatures and thus had a recrystallization temperature in excess of 3700 F.

Example 6 Using the procedure of Example 3 an alloy of the following composition was prepared: 1% yttrium, the balance being tantalum. The amount of yttrium initially added to the melt was 1% by weight. This alloy had a recrystallization temperature of 3400 F. and an ASTM grain size of 8. When the alloy was heated at 3700 F. for one hour, it had an ASTM grain size of 6.

The advantages of the present invention as attained by the addition of yttrium metal to tantalum metal as demonstrated in Examples 1-5 can be readily apperciated when compared to the recrystallization temperature and grain size of tantalum metal without the yttrium addition. Pure tantalum metal has a recrystallization temperature of 2150 F. and an ASTM grain size of 6. When pure tantalum metal in the form of .020 inch thick sheet and wire was heated to 3700 F., the individual grains extended across the thickness of the wire and sheet. The grain size of the pure tantalum as 3700" F. was much greater than AST M 0.

Example 7 Using the procedure of Example 3 an alloy of the following composition was prepared: 8% tungsten, 2% hafnium, 02% yttrium, the balance being tantalum. The the amount of yttrium initially added to the melt was .05 by weight. This alloy had a recrystallization temperature of between 3250 F. and 3700 F. When this alloy was heated to a temperature of 3720 F., it had a grain size of 7-8. In contrast, a tantalum 8% tungsten 2% hafnium alloy without the yttrium addition had a recrystallization temperature of 3250 F. and an ASTM grain size of 7-8. This alloy when heated at 3720 F. for one hour had an ASTM grain size of 5-8.

Example 8 Using the procedure of Example 3 an alloy of the following composition was prepared: 10% tungsten, .0l% yttrium, the balance being tantalum. The amount of yttrium initially added to the melt was .2%. This alloy had a recrystallization temperature of 3250 F. and an ASTM grain size of 7-8. This alloy when heated at 3720 F. for one hour had a grain size of 4-8. In contrast, tantalum 10% tungsten alloy without the yttrium addition had a recrystallization temperature of 2800- 2900 F. and an ASTM grain size of 5-8.

When the Ta-IOW alloy Without the yttrium addition was heated at 3720 F. for one hour, it had an ASTM grain size of l-2.

Example 9 Using the procedure of Example 3 an alloy of the following composition was prepared: 30% columbium, 7 /2% vanadium, .03% yttrium, the balance being tantalum. The amount of yttrium initially added to the melt was .05 This alloy was recrystallized at a temperature of 2750 F. and had an ASTM grain size of 8. When heated to a temperature of 3720 F. the alloy was recrystallized and had a grain size of l-3. In contrast a tantalum 30% columbium, 7 /2% vanadium alloy without the yttrium addition had a recrystallization temperature of 2500 to 2750 F. and an ASTM grain size of 8. When this alloy was heated at 3720 F. for one hour, it had an ASTM grain size of 1 and larger.

While the invention has been described initially in connection with the use of yttrium metal additions to tantalum and tantalum-base alloys, the effect obtained by yttrium on recrystallization temperature and grain growth is also found in additions of yttrium oxide and the rare earths (including lanthanum) and their oxides.

Example 10 Using the procedure of Example 3, a number of alloys containing other rare earth and their oxides was prepared.

The results of this series of experiments are given in Table I below. In this table the Initial alloy composition is the amount of alloying constituent added to the tantalum chips at the start of the button-melting operation. The Final analysis is the amount of alloying constituent determined in the final sheets by spcctrographic methods. In the following table, R means rocrystallized. The numbers are the grain sizes, the larger the number, the smaller the grain size. Tests on pure The yttrium-containing tantalum isat least five times tantalum are included in this table as well for comparison stronger than the pure tantalum and the strength of ytpurposes.

TABLE I.-ANNEALING BEHAVIOR OF TANTALUM CONTAINING RARE EARTH METALS AND OXIDES Grain Size and Percent Recrystallization Alter Annealing 1 Hour at the Indicated Initial Alloy Final Analysis, Temperature, F. Composition ppm. Rare Earth Tantalum.

50% R Approx. 100% R Approx. 500... 50% R 75 0 Approx. 500...

Til-0.3 Ndz03 Ta-l Ndzoa 11 Many blocky grains have enlarged rather than forming true equiaxed grains.

Example 11 trium-containing tantalum at 2000 C. is higher than Alloys containing yttrium oxide (Y O' were prethe strength of pure tantalum at 1650 C. This supepared by the procedure of Example 3. Annealing reriority of the yttrium-containing tantalum is again demsults are given in Table II.

TABLE II.ANNEALING BEHAVIOR OF TANTALUM CONTAINING YTTRIUM OXIDE Final Grain Size and Percent Recrystallization After Annealing 20 Minutes Initial Analysis, at the Indicated Temperature, F. Alloy p.p.m. Composition Rare Earth Ta0.05 Yzos 55 75-100% R, 58 85% R, 4-8 R, 3-8 B 1-8, Avg. 5-6 Ta-0.2 Y O 250 75% R, 3-8 B 2-8, Avg. 4-5 R 1-8, Avg. 4-5-- R 1-8, Avg. 4-5. Ta0.5 YzO 94 B 1-8, Avg. 45 R 1-8, Avg. 4-5.. B 1-8, Avg.4-5 B 1-8, Avg. 4-5.

onstrated in the stress rupture behavior. These results In addition to the effect of yttrium on grain growth are given below and recrystallization temperature in tantalum, it has been A discovered that alloys of the type produced by arc melt- T BLE IV ing tantalum with small additions of yttrium give greatly Temperature, Stress, Rupture increased tensile strength and creep resistance at ele- Material "0. psi. Time, vated temperatures. Minutes Example 12 1, 050 2,550 51 Several commercial scale ingots were produced in ac- Tantalum @838 2%? 8 cordance with the techniques of Example 1, the first, heat 21000 2, 450 170 3,405 (30 p.p.m. yttrium) 2 000 1 500 450 3405, contained approximately 30 p.p.m. yttrium, and 55 11650 71480 315 the second, heat 3112, contained approximately 80 ppm. 3, ppy 2,000 2,630 3 0 yttrium. These ingots were forged to 1-inch sheet bar 20O0 500 597 and then rolled directly to Az-inch plates. The plates were next annealed for about 3 hours at 2200" F. and The yttnum'contammg tantalum alloys at 29000 then rolled to 0.030 sheet. These two samples of comt to tantalum at The matenal Wlth merical-scale yttrium-containing tantalum sheets were hgher Yttnum content h slgmficantly better Tupture then tested against pure tantalum sheets. These results Properties than the matenal with the lower Yttrium are set forth below in Tables III and IV. tent' Since certain changes may be made in the above products and process without departing from the scope of the invention herein involved, it is intended that all mat- TABLE III.HOTTENSILE STRENGTH OFTANTALUMAND ter contained in the above description shall be i-nter- YTTRIUMCONTAINING SHEET preted as illustrative and not in a limiting sense.

a What is claimed is: Temperature Tantalum (30 35%. Y) 80531 131 1. A cold Worked tantalum alloy having a fine grain structure and being resistant to grain coarsening upon 1,650 u g8 3 238 annealing, said alloy having an ASTM number greater 2000 660L990 1666- 5.260 than 3 upon heat ng to 3700 F. for one hour and havmg increased tensile strength and creep propertles as com- 2 The ditference between 660-990 is a. load difiference of gaz g gg ii ilevated g ggq Sald poundm y g e on rat y o a rare ea a itive selected from the group consisting of elements having atomis numbers 39 and 57 through 71, mixtures of such elements and oxides thereof, said additive being greater than p.p.m. and less than 0.1 weight percent of said alloy, the balance consisting essentially of tantalum metal and tantalum-base alloys containing at least 60 weight percent tantalum, the balance of said tantalum base alloy consisting predominantly of at least one metal selected from the group IV, V and VI metals and being essentially free of additional elements having adverse elfects on the high temperature properties of tantalum alloys.

2. A cold worked alloy according to claim 1 having an yttrium content greater than 10 p.p.m. and less than 0.1 weight percent of said alloy, the balance being tantalum, said alloy having a tensile strength in excess of 4000 p.s.i. at 2000 C.

3. An alloy according to claim 1 having an yttrium content greater than 10 p.p.m. and less than 0.1 Weight percent of said alloy, 8% tungsten, 2% hafnium, the balance being tantalum.

4. An alloy according to claim 1 having an yttrium content greater than 10 p.p.m. and less than 0.1 weight percent of said alloy, 10% tungsten, the balance being tantalum.

5. An alloy according to claim 1 having an yttrium content greater than 10 p.p.m. and less than 0.1 weight percent of said alloy, 30% columbium, 7 /2% vanadium, the balance being tantalum.

6. The cold worked tantalum alloy of claim 1 wherein the additive consists essentially of those additives selected from the group consisting of cerium and cerium oxide.

7. The cold worked tantalum alloy of claim 1 wherein the additive consists essentially of those additives selected from the group consisting of lanthanum and lanthanum oxide.

8. The cold worked tantalum alloy of claim 1 wherein the additive consists essentially of those additives selected from the group consisting of neodymium and neodynium oxide.

9. The cold worked tantalum alloy of claim 1 wherein the additive consists essentially of those additives selected from the group consisting of mischmetal.

References Cited by the Examiner UNITED STATES PATENTS 2,187,630 1/1940 Schafer -174 3,028,236 4/1962 Wlodels 75-174 3,037,858 6/1962 Weisert 75-174 3,156,560 11/1964 Semmel 75-174 FOREIGN PATENTS 323,315 6/1932 Canada.

OTHER REFERENCES Metals Reference Book, 2nd ed., vol. 1 Smithells, Butterworths Scientific Publications, London, 1955, pp. 179-180.

N b-Ce phase diagram, found in Rare Metals and Their Alloys, published by E. M. Savitskiy in Dom. Tekhniki, Moscow, 1959, one page.

Nuclear Science and Engineering, Bidwell et al., vol. 14, pages l09-122, 1962.

Structure of Metals, Barrett, McGraw-Hill, Inc., N.Y., 1943, pp. 205-208. V

Structure of Metals and Alloys, Hume-Rotherty, published by the Institute of Metals, London, 1947, pp. 59-63, 66-68.

WADD TR 5913, Investigation of the Properties of Tantalum, and its Alloys, Schmidt et al. (BMI), March 1960, pp. 37-38.

DAVID L. RECK, Primary Examiner.

W. C. TOWNSEND, Examiner.

C. N. LOVELL, Assistant Examiner. 

1. A COLD WORKED TANTALUM ALLOY HAVING A FINE GRAIN STRUCTURE AND BEING RESISTANT TO GRAIN COARSENING UPON ANNEALING, SAID ALLOY HAVING AN ASTM NUMBER GREATER THAN 3 UPON HEATING TO 3700*F. FOR ONE HOUR AND HAVING INCREASED TENSILE STRENGTH AND CREEP PROPERTIES AS COMPARED TO PURE TANTALUM AT ELEVATED TEMPERATURES, SAID ALLOY CONSISTING ESSENTIALLY OF A RARE EARTH ADDITIVE SELECTED FROM THE GROUP CONSISTING OF ELEMENTS HAVING ATOMIC NUMBERS 39 AND 57 THROUGH 71, MIXTURES OF SUCH ELEMENTS AND OXIDES THEREOF, SAID ADDITIVE BEING GREATER THAN 10 P.P.M. AND LESS THAN 0.1 WEIGHT PERCENT OF SAID ALLOY THE BALANCE CONSISTING ESSENTIALLY OF TANTALUM METAL AND TANTALUM-BASE ALLOYS CONTAINING AT LEAST 60 WEIGHT PERCENT TANTALUM, THE BALANCE OF SAID TANTALUM BASE ALLOY CONSISTING PREDOMINANTLY OF AT LEAST ONE METAL SELECTED FROM THE GROUP IV, V AND VI METALS AND BEING ESSENTIALLY FREE OF ADDITIONAL ELEMENTS HAVING ADVERSE EFFECTS ON THE HIGH TEMPERATURE PROPERTIES OF TANTALUM ALLOYS. 